Alexandra E. Raevskaya

Thin films of colloidal CZTS nanocrystals (NCs) synthesized using a “green” approach in water with a variation of the copper-to-tin ratio are investigated by Raman scattering, mid-infrared (molecular vibrations) and near-infrared (free carrier) absorption, X-ray photoemission spectroscopy (XPS), electrical conductivity, and conductive atomic force microscopy (cAFM). We determined the effect of the actual Cu content on the phonon spectra, electrical conductivity, and spectral parameters of the plasmon band. An increase in the electrical conductivity of the NC films upon annealing at 220 °C is explained by three factors: formation of a Cu_xS nanophase at the CZTS NC surface, partial removal of ligands, and improved structural perfection. The presence of the Cu_xS phase is concluded to be the determinant factor for the CZTS NC film conductivity. Cu_xS can be reliably detected based on the analysis of the modified Auger parameter of copper, derived from XPS data and corroborated by Raman spectroscopy data. Partial removal of the ligand is concluded from the agreement of the core-level XPS and vibrational IR spectra. The degree of lattice perfection can be conveniently assessed from the Raman data as well. Further important information derived from a combination of photoelectron and optical data is the work function, ionization potential, and electron affinity of the NC films.

A general synthesis approach of aqueous glutathione-capped ternary Ag–In–S, Cu–In–S, and Hg–In–S nanocrystals (NCs) is introduced, allowing the NC composition to be varied in a broad range. Ternary Hg–In–S (HIS) NCs are reported for the first time and found to have the same tetragonal chalcopyrite motif as Cu–In–S and Ag–In–S NCs, corroborated by phonon spectra, while X-ray photoelectron spectroscopic data indicate mercury to be present as Hg⁺ in the Hg–In–S NCs. Colloidal HIS and Hg–In–S/ZnS NCs showed little or no variations of the spectral width of the photoluminescence band upon NC size selection, temperature variation in a broad range of 10–350 K, deposition of a ZnS shell, or postsynthesis annealing. All these observations are similar to those reported earlier for Ag–In–S and Ag–In–S/ZnS NCs and allowed us to assume a general photoluminescence mechanism for all three ternary compounds, based on the model of radiative self-trapped exciton recombination.

The effect of flash-lamp annealing (FLA) on the re-crystallization of thin films made of colloidal Cu₂ZnSnS₄ nanocrystals (NCs) is investigated by Raman spectroscopy. Unlike similar previous studies of NCs synthesized at high temperatures in organic solvents, NCs in this work, which have diameters as small as 2–6 nm, were synthesized under environmentally friendly conditions in aqueous solution using small molecules as stabilizers. We establish the range of FLA conditions providing an efficient re-crystallization in the thin film of NCs, while preserving their kesterite structure and improving their crystallinity remarkably. The formation of secondary phases at higher FLA power densities, as well as the dependence of the formation on the film thickness are also investigated. Importantly, no inert atmosphere for the FLA treatment of the NCs is required, which makes this technology even more suitable for mass production, in particular for printed thin films on flexible substrates.

The paper reviews the current state in synthesis, photovoltaic, photocatalytic and photoelectrochemical applications of multinary metal chalcogenide nanocrystals and nanostructures.

Colloidal size-selected copper-doped Ag–In–S quantum dots were produced directly in aqueous solutions by fractionation/redispersion with a plethora of emission colors and a top luminescence quantum yield of around 60%.